An analytic formulation of transport in isolated renal tubules is proposed. By these means numerous parameters and variables in the transport mechanism can be isolated for individual study by experimental procedures including in vitro microperfusion of single nephron segments. If successful, the analysis proposed here will add greatly to the understanding of nephron function, will serve as a guide for future research in renal tubule physiology, and will suggest new means by which normal and abnormal kidney transport phenomena can be manipulated in a clinical setting. Because transport mechanisms employed by the renal tubule may have features in common with transport in many other organs, the analytic technique developed in this proposed research may have far-reaching applications in many areas of medicine and biology.